This invention relates to a pulse counter having an arbitrary monotonic function as an output characteristic.
A pulse counter receives a clock pulse signal and generates a digital output signal having a value related to the number of clock pulses input. In the simplest case the counter increments once for each input pulse, thus counting the number of input pulses; the output characteristic of this counter can be represented by a straight line with unity slope. Alternatively, the counter may be adapted to increment once every N input pulses; in this case the output characteristic is a straight line with a slope of 1/N. For many applications, however, a non-linear output characteristic such as a quadratic or exponential function is required.
Japanese Patent Application Kokai Publication No. 14758/1980 discloses a pulse counter comprising a read-only memory (ROM) and a latch. The latch receives the clock signal, latches low-order bits of the ROM output, and feeds these bit values back to the ROM as address inputs. The output of the pulse counter comprises high-order bits of the ROM output. By programming the ROM with appropriate data, the output characteristic of this pulse counter can be made to approximate an arbitrary mathematical function. A problem, however, is that every output value must be stored in advance in the ROM. To obtain a smooth output characteristic the ROM must store a large amount of data, requiring not only a large number of memory cells but also a large number of address signal lines and a large latch.
The amount of circuitry required can in some cases be reduced by cascading pulse counters of the above type, but this tends to produce another problem: sudden jumps in the output characteristic at points at which the output characteristic changes slope, occurring because the weights of the outputs of the counter stages are not always powers of two.